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Agilent
Calibrating Standards
for In-Fixture Device
Characterization
White Paper
In many products such as PCS and cellular phones, the
continuing miniaturization of components and subsystems Standards (SOLT)
has all but eliminated the use of coaxial connectors as an
internal interconnection method. In these products, a
bandpass filter may be only a few centimeters long, and
is mounted directly to the PC board, without connectors. DUT
Measurement
This presents a problem when using a vector network plane
analyzer to evaluate the characteristics of a component,
since there is no longer a well-characterized interface
(the connector of the device under test) to which the
analyzer can connect.
Figure 1(a). A bandpass filter test fixture
The answer is the test fixture, which is a good solution
as long as it is well constructed, its characteristics
are known, and its effects can be removed from the
measurement results. The fixture must be calibrated,
usually by means of the short-open-load-through (SOLT)
calibration technique. Calibration of a fixture used to
evaluate bandpass filters for mobile phones provides
a good example of the details that must be considered
and the process itself. The accuracy of in-fixture network
measurements is directly related to the process used to
calibrate the fixture.
Figure 1a shows an example fixture for testing a bandpass
filter. The fixture's SMA connectors are the interface to
the network analyzer, and "pogo" type connectors connect
to the filter under test. Its characteristics in the time
domain with the through standard in place are shown
in Figure 1b. Transitions in the fixture are readily
identifiable -- and markers 1 and 4 show the transition
at the SMA input and output connectors respectively,
and markers 2 and 3 show the transition of the input Figure 1(b). Characteristics in the time domain with the through standard
and output "pogo" connectors. Between markers 2 and 3 in place of the filter shown in Figure 1(a)
is the reflection coefficient of the through standard, which
can be used to calculate the transmission line impedance.
2
In the time domain, a network analyzer's gating function In-fixture Standards
can be used to remove all data from the measurement
except that obtained from the fixture. The match of the A set of in-fixture standards consists of a short, open,
fixture may then be analyzed in the frequency domain load, and through, and is the same size as the DUT
with gating on (Figure 2a). In this case, the gate starts so that they may be inserted into the fixture during
at the SMA transition and stops at the input "pogo" calibration. This also allows the "pogo" pins to be
connector. Figure 2b shows the frequency response compressed the same amount for both the standards
of the fixture with gating on. The match at 2 GHz is and the DUT, which helps define the measurement plane.
about 25 dB. If the match of the filter is 20 dB, then the Defining the measurement plane is a key ingredient in
measurement uncertainty will be high, often manifesting the calibration process, because it is the point at which
itself as ripple in the data trace. the analyzer makes its measurement. Consequently,
careful determination of this point ensures that
undesired electrical characteristics that occur before
the measurement plane are not included in the results.
The measurement plane should ideally be at the RF
connections of the DUT.
The short standard is a block of conductive material and
the open standard is a non-conductive dielectric block.
The load standard consists of two 100-ohm resistors in
parallel, connected to a short microstrip line that ends
in a contact pad, which the "pogo" pins contact when
inserted into the fixture. In this case, the pins are only
touching (contacting) the pad. The use of parallel resistors
reduces the series inductance, thereby enhancing the
performance of the load element. The through standard
is a microstrip transmission line that connects the two
"pogo" pins together when inserted into the fixture.
The characteristics of the calibration standards must
be determined, and this electrical data (which forms the
Figure 2(a). The match of the fixture viewed in the time domain with calibration kit definition) must be input to the network
gating on analyzer in order for it to perform the required error
correction. This calibration data includes values of
impedance, frequency, loss, delay, fringing capacitance,
and inductance. For example, the open standard may
be offset from where it interfaces to the fixture, so this
information is entered as offset delay, offset impedance,
and offset loss. An open standard may also have "fringing"
capacitance at the open connection, which must be
included as well. The other standards have similar
characteristics that must be measured and input to
the analyzer.
Figure 2(b). Frequency response of the fixture viewed in the time domain
with gating on
3
Characterizing the standards
The first step is to perform a calibration at the point
where low-loss flexible microwave cables terminate in
the connectors that will mate to the test fixture. The
calibration must be performed with the proper calibration
kit and associated calibration kit definition in the
analyzer. For this calibration, the Agilent PNA-L vector
network analyzer and 85052D calibration kit and
definition file can be used. The fixture is then connected
to the analyzer and a marker is placed at 1 GHz. Since
the offset delay equation requires that insertion loss of
the through standard be measured at 1 GHz, the remaining
measurements were also made at this frequency for
consistency. The terms of the open standard (C0 through
C3) have negligible impact at this frequency.
The analyzer setup is as follows:
Start frequency 50 MHz
Stop frequency 20.05 GHz Figure 3. The Smith chart when the short standard is electrically
Number of points 401 shorter than the open. The trace rotates backward
Time domain mode Low-pass step
Calibration 2-port SOLT
If this occurs, the offset length must be adjusted using
A short standard is defined as having unity reflection the port extensions for port 1, as was done for the short
and 180 degree phase shift, and it defines where the standard, until the phase response is monotonically
measurement plane resides. The short standard is negative. The difference between the value of the port
inserted into the fixture, the analyzer is set to measure extension for the short and for the open will be a negative
S11, and the format is set to phase. value since the port extension was reduced for the open.
This negative value will be entered into the calibration
The port extensions for port 1 are then adjusted until the kit definition as an offset length in picoseconds. With
phase reads 180 degrees at the marker. It may be helpful this new offset in place, the Smith chart (Figure 4) now
to set the reference value of the display to 180 degrees to displays capacitance instead of inductance.
avoid jumps from